Exporting / importing .obj messes up the materials
You cannot avoid some changes, simply because .OBJ (.MTL actually) shading model is not exactly the same as BI one (not talking about nodes here, if it’s relatively easy to import .mtl as a basic nodal shader, the reverse operation is practically impossible and hence not supported).

That said, several issues here were fixed in past months (regarding mutual influences between world and material especially), so suggest you try with the latest build from our buildbot, if not yet done. :)